Color Patch: Go Green

23. August 2016

Burn patients often receive antibiotics as a precaution. It's not always so that current tests confirm the presence of infection. In order to avoid unnecessary antibiotic treatment, researchers have developed a wound dressing that alters its colour once the wound is infected.

Shortly after it originates, a wound is colonised by bacteria from the surrounding skin or environment – present among these are also opportunistic pathogens. These pathogens occur ubiquitously and can cause disease in people with a weakened immune system or other physiological deficits. In many cases, after the colonisation of the wound, the bacterial population remains low, so that the wound healing is not further restricted. Antibiotic therapy is not necessary in this case.

Sometimes, however, the bacteria multiply so intensely that the patient’s immune system is no longer able to “keep it in check”. Should the pathogens penetrate into the tissue, a clinically significant infection results. An extremely high risk for wound infections is present in burn wounds. This is because they often take up a large area and due to immunological complications that follow the thermal injury they are vulnerable. Moreover it is difficult even today for doctors to determine at the patient’s bedside whether an infection is present. This is because symptoms such as fever may result from a bacterial infection, yet this need not be the case. Procedures through which a reliable diagnosis is made possible can take up to 48 hours. By then, however, complications such as sepsis, shock or death may have occurred. Patients often for this reason as a precaution first receive antibiotic treatment.

Problem: Antibiotic resistance

Through the frequent use of antibiotics in hospitals, but also in agriculture, bacteria develop resistances such that medications become ineffective. This so-called secondary resistance can develop due to genetic events, such as mutation or transfer of resistance genes, and subsequent selection. An important approach in order to prevent drug resistance is, therefore, to use antibiotics only selectively. “If we do not act in the fight against antimicrobial resistance soon, it will mean the end of modern medicine”, the English Chief Medical Officer Dame Sally C. Davies 2015 at the World Heath Summit stated. The World Health Organisation is already warning of a “post-antibiotic” era, in which ordinary infections and minor injuries could end fatally.

Medical bandage changes colour

A medical dressing which changes colour once the wound is infected will help to avoid unnecessary antibiotic treatment and to thereby detect infections at an early stage. It was developed by scientists led by Toby Jenkins [Paywall] from the University Bath in the United Kingdom, in collaboration with the Children’s Burns Research Centre at the Children’s Hospital in Bristol and the University of Brighton. “Our medical dressing works by releasing a fluorescent dye from nanocapsules. This release is triggered by toxins which the disease-causing bacteria in the wound secrete”, explains project leader Toby Jenkins.

The nanocapsules mimic skin cells. They only break up when toxic bacteria are present. Bacteria which live on healthy skin and are harmless have no effect on the capsules. This allows doctors – without removing the dressing – to quickly determine whether a wound is infected or not. “It can really help to save lives”, Toby Jenkins says euphorically. Brian Jones from the molecular and medical microbiology specialty section of the University of Brighton adds: “This new dressing technology will not only help doctors in providing patients with the best possible burns care, but could also tell us a lot about how wound infections start and how they interfere with the normal healing process. This in turn could lead to further progress in the treatment of infections”.

The study

The wound dressing used by Toby Jenkins’ research team consists of a hydrogenated agarose film, in which a mixture made up of vesicles and fluorescent dye and agarose is spread. The research team used 5,6-carboxyfluorescein as a dye. This substance forms dimers when present over a certain concentration. A dimer is a molecule that consists of two identical or only slightly different blocks. Within the nanocapsule, the dye is present in concentrated form, so it forms dimers and does not fluoresce (self-quenching). However if it is released, it becomes diluted and fluoresces green. The sensitivity of the bandage depends on the pathogen strain. The better the pathogenic bacteria forms biofilms, the greater this sensitivity is. Biofilms are thin mucus films in which the pathogens are present in organised form. This protects the bacteria from the immune system as well as from antimicrobials such as antibiotics. Biofilms are formed in more than six out of ten bacterial infections.

The dressing was tested using the biofilm and the culture supernatants of the bacteria which most commonly cause wound complications – namely Staphylococcus aureus, Pseudomonas aeruginosa and Enterococcus faecalis. The researchers thereby inoculated a polycarbonate membrane with the respective bacteria and let them grow for a certain time under specified conditions. Negative controls used by the research team were HEPES buffer and non-pathogenic Escherichia coli. Both Staphylococcus aureus and Pseudomonas aeruginosa showed a strong and clear fluorescence activity, which was absent with the negative controls and with the pathogenic Enterococcus faecalis.

Both the structure as well as properties of the biofilm depend on the physical environment. For this reason, the research team next tested two Staphylococcus aureus-biofilms. One had been formed by the bacteria at rest, ie. formed under static conditions, in an environment with low shear effects. The other was created under dynamic environmental conditions in flowing medium. If the dressing came into contact with the “static biofilms”, the scientists observed that, compared with the “dynamic biofilms” of similar age, there was better fluorescence.

However, as the authors write, the biofilm was not examined more closely, so that the differences may have resulted from a different film thickness. Since this model also does not entirely correspond to the actual conditions present in the wound, the group led by Toby Jenkins then tested the medical dressing on thermally damaged pigskin tissue. 24 hours after application of the bacteria, the researchers were able to detect the colonisation of the wound and a jelly-like film. If the wound dressing was then applied to the tissue, in all infected wounds – therefore also in the wound treated with Enterococcus faecalis – fluorescing started within six hours. The negative controls, however, remained colourless.

What’s next?

Scientists plan to test the dressing on samples obtained from patients with burn wounds. For this purpose, the group has received about one million British pounds from the Medical Research Council. In about four years the wound dressing should then be available in the market. In addition, the group led by Toby Jenkins wants to develop a wound dressing which, depending on which pathogen causes the wound infection, takes on differing colours.

Dr. Amber Young (left) and Dr. Toby Jenkins test the dressing prototypes based on samples from the wounds of burns victims test (source: University of Bath)

The risk of bacterial infection is high not only in burn wounds. “Catheter infections are such a widespread problem that currently anyone who has a catheter for more than seven days receives antibiotic therapy […]”, Toby Jenkins says. For this reason scientists developed a chemical coating which can be used with existing catheters. It consists of two layers. The first layer dissolves when due to infection from Proteus mirabilis the urine turns alkaline. The underlying layer consists of a gel with a non-toxic dye. If this is released, the dye can pass into the urine bag, causing it to turn bright yellow.